Dennis Pitt (Anthony Perkins) is a mentally disturbed guy who is released from years of imprisonment. He becomes interested in a high school girl, Sue Ann Stepanek (Tuesday Weld) and invents a story of how he's really a secret agent on a mission. He's working at a local chemical plant but convinces her it's simply his cover. But things go awry when it turns out she's actually more crazy than he is, especially when he sees that she has no problem killing people.
This is a great character study of two messed up characters. The unpredictable script is a lot of fun, considering the subject matter. Highly recommended, especially if you're weary of the past twenty-years of lame serial killer movies.